Infra-red Spectroscopy CHM 504 Infra-red Spectroscopy

Electromagnetic Spectrum

Objectives of IR spectroscopy To identify the functional groups present in molecules To identify compounds by comparison with spectra of known compounds in a database.

Molecular vibrations Natural frequency of vibration depends on Mass of each atom; light atoms (e.g., H) vibrate faster. Strength of bond; strong bonds vibrate faster.

Modes of vibration Molecules with more than two atoms can vibrate in several different ways. Each way is called a MODE. Every mode of vibration has a natural frequency. Each mode of vibration involves distortions of Bond length : STRETCHING Bond angle : BENDING

Vibrations of H2O Symmetric stretching Asymmetric stretching Bending

Interaction of matter with infra-red energy Molecules can absorb energy in the form of infra-red radiation. The radiation being absorbed must have the same frequency as one of the modes of vibration of the molecule. When a molecule absorbs one photon of IR radiation of appropriate frequency, the corresponding mode of vibration increases its amplitude. The frequency of vibration does not change.

Vibrations and infra-red radiation Amplitude of vibration increases Frequency of vibration unchanged

Energy of IR radiation E = hn l n = c IR radiation is a form of electromagnetic radiation. Every photon of electromagnetic radiation has a quantum of energy, given by the equation E = hn where h is Planck’s constant, 6.626  10-34 J s. The frequency n is related to the wavelength l by the equation l n = c where c is the speed of light.

Measurement of IR radiation Can be measured as either frequency (cycles per second, Hz, s-1) or wavelength (micrometers, mm). For historic reasons, usually measured as wavenumber (cycles per centimeter, cm-1). Wavenumber is a kind of frequency; the greater the wavenumber, the higher the energy. Symbol for wavenumber is .

Wavenumber and wavelength Absorption of IR radiation due to vibrations occur in the range 10 000 - 100 cm-1 (1 - 100 mm) Range of real interest in IR spectroscopy: 4000 - 400 cm-1 (2.5 - 25 mm)

Infra-red Spectrum Graph Horizontal axis: Vertical axis: Wavelength (2.5 - 25 mm) OR Wavenumber (4000 - 400 cm-1) – usually. Vertical axis: Absorbance (A) OR Transmittance (T) – usually (0 - 100%).

IR spectrum of 3-hydroxyacetophenone

Some definitions Transmittance: Absorbance:

Spectral bands or peaks A complex molecule has many modes of vibration. Each mode has a characteristic frequency. The molecule can absorb IR radiation of each characteristic frequency. Each such absorption appears as a band or peak in the IR spectrum.

Identification of molecules Each molecule has a unique set of modes of vibration. Therefore each molecule has a unique spectrum. Unknown molecule can be identified by comparing spectrum with spectra of known molecules. Exact one-to-one matching of peaks sufficient to identify molecule. Computer required to search database of known spectra.

Qualitative information from IR spectra Many modes of vibration principally involve specific bonds or functional groups. Peaks corresponding to those vibrations reveal the presence of those functional groups. Each peak has 3 characteristics that can provide information: Wavenumber Intensity (strong, medium, or weak) Shape (sharp, normal, or broad) Not every peak provides useful information.

Shapes of peaks sharp normal broad

Analysis of IR spectrum Spectrum can be divided into 4 regions: Region 1: 4000 - 2550 cm-1 Region 2: 2500 - 2000 cm-1 Region 3: 1900 - 1400 cm-1 Region 4: 1400 - 400 cm-1 Different types of vibrations, corresponding to different functional groups, are found in different regions.

4000 - 2550 cm-1 C–H, N–H, O–H, and (rarely) S–H stretching vibrations. Can be distinguished based on wavenumber, intensity, and shape. Very important in the identification of Alcohols / phenols Carboxylic acids Primary / secondary amines Amides Terminal alkynes (R–CC–H)

2500 - 2000 cm-1 Triple bond stretching (R–CC & R–CN) Stretching in cumulative pairs of double bonds (X=C=Y, where X and Y could be C, N, or O). In most spectra, this area is blank.

1900 - 1400 cm-1 C=C, C=O, and C=N stretching peaks. Stretching of bonds that are intermediate between single and double (1300 - 1600 cm-1). For example, Often the most important part of the spectrum

Functional groups visible in 1900 - 1400 cm-1 region Aldehydes and ketones Other C=O containing functional groups, including Carboxylic acids Acid chlorides and anhydrides Esters Amides C=C double bonds Aromatic rings

1400 - 400 cm-1 Stretching of single bonds to atoms other than H, e.g., C–C, C–O, C–N, C–Cl, etc. C–H bending peaks. Usually lots of peaks. Most cannot be identified or interpreted. Region of spectrum that tends to be unique for a given compound. Sometimes referred to as the fingerprint region.

Identification of functional groups Based on presence of key peaks in spectrum. Wavenumber, shape, and intensity of peak should be considered. Some functional groups cannot be easily identified using IR, e.g., those containing only single bonds other than O-H, N-H. Alkyl halides Ethers Tertiary amines

Alkanes (alkyl groups) C–H stretching: 2850 - 2960 cm-1 C–H bending: 1150 - 1390, 1450 - 1465 cm-1 Many overlapping bands Presence of these peaks is not informative, since most compounds contain alkyl groups.

Alkenes Three types of diagnostic peaks. 1. C=C stretching (1640 – 1670 cm-1, w to m) 2. C–H stretching (3000 – 3100 cm-1) 3. C–H bending

IR spectrum of 1-hexene

Alkynes 1. CC stretching. 2. C–H stretching. 3. C–H bending. R–CC–H 2140 - 2100 cm-1 (medium) R–CC–R’ 2260 - 2190 cm-1 (weak or absent) 2. C–H stretching. 3330 - 3270 cm-1 (strong, sharp) 3. C–H bending. A terminal alkyne (R–CCH) is easily recognised; an internal alkyne is very difficult to spot using IR.

IR spectrum of 1-hexyne

Aromatic hydrocarbons 1. C=C stretching: two sets of peaks. (i) 1600 - 1585 cm-1 (weak - medium) (ii) 1500 - 1400 cm-1 (weak - medium) Each set typically contains two peaks. The second peak in each set may be weak, absent, or appear as a “shoulder.” 2. C–H stretching. 3100 - 3000 cm-1. (indistinguishable from alkene C-H stretch) 3. C–H bending

IR spectrum of toluene

Alcohols and phenols 1. O–H stretching. 3550 - 3200 cm-1 (strong, broad) Broad because of H – bonding. In dilute solutions, sharp peak at ~3600 cm-1. 2. C–O stretching. 1260 - 1000 cm-1 (strong) (Cannot distinguish between alcohols and phenols based on IR)

IR spectrum of (CH3)2CHCH2OH

The carbonyl group

Identifying a carbonyl-containing functional group C=O stretching peak Very strong peak - often the strongest in the spectrum Consider other characteristic peaks O–H stretching of carboxylic acid C–H stretching of aldehyde N–H stretching and bending of amide C–O stretching of ester Absent such peaks, probably a ketone

Aldehydes C=O stretching. 1680 – 1740 cm-1 C–H stretching. 2830 - 2690 cm-1 1-2 peaks, relatively weak No other peaks appear in this range

Ketones C=O stretching : 1670 – 1750 cm-1 Can be distinguished from aldehydes by absence of C–H peaks at 2830 - 2690 cm-1.

IR spectrum of CH2CH2CH2CHO

IR spectrum of CH3C(O)CH2CH2CH3

Carboxylic acids C=O stretching. 1680 – 1720 cm-1 O–H stretching: very broad, distinctive peak, not very strong, stretching from ~3300 to ~2500 cm-1 (centered at ~3050 cm-1); C–H stretching peaks are usually superimposed.

IR spectrum of CH3(CH2)4CO2H

Esters C=O stretching. 1715 – 1770 cm-1. A very strong peak. Absent in ketones; can be used to distinguish between esters and ketones. However: its presence does not guarantee an ester! Could be a ketone with a C–O single bond elsewhere.

IR spectrum of CH3C(O)OCH2CH3

Amides C=O stretching: 1680 – 1630 cm-1. N–H stretching: 3400 – 3180 cm-1. Primary amides: 2 peaks Secondary amides: 1 peak Tertiary amides: missing

IR spectrum of CH3CONH2

Primary and secondary amines N–H stretching: 3400 - 3250 cm-1 Primary (RNH2) : 2 peaks Secondary (R2NH) : 1 peak Relatively weak and sharp. Much weaker than amide N–H stretching. Easy to distinguish from strong, broad OH stretching. May be obscured if OH, NH2 in same molecule.

IR spectrum of CH3(CH2)3NH2